<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <meta http-equiv="X-UA-Compatible" content="ie=edge" />
    <title>Document</title>
    <style>
      .placeholder {
        background-color: #f6f6f6;
        background-size: cover;
        background-repeat: no-repeat;
        position: relative;
        overflow: hidden;
      }
      .placeholder img {
        position: absolute;
        opacity: 0;
        top: 0;
        left: 0;
        width: 100%;
        transition: opacity 1s linear;
      }
      .placeholder img.loaded {
        opacity: 1;
      }
      .img-small {
        filter: blur(50px);
        /* this is needed so Safari keeps sharp edges */
        transform: scale(1);
      }
    </style>
  </head>

  <body>
    <div
      class="placeholder"
      data-large="https://cdn-images-1.medium.com/max/1800/1*sg-uLNm73whmdOgKlrQdZA.jpeg"
    >
      <img
        src="https://cdn-images-1.medium.com/freeze/max/27/1*sg-uLNm73whmdOgKlrQdZA.jpeg?q=20"
        class="img-small"
      />
      <div style="padding-bottom: 66.6%;"></div>
    </div>
    <script>
      window.onload = function() {
        const placeholder = document.querySelector('.placeholder'),
          small = placeholder.querySelector('.img-small')
        // 1: load small image and show it
        const img = new Image()
        img.src = small.src
        img.onload = function() {
          small.classList.add('loaded')
        }
        // 2: load large image
        const imgLarge = new Image()
        imgLarge.src = placeholder.dataset.large
        imgLarge.onload = function() {
          imgLarge.classList.add('loaded')
        }
        placeholder.appendChild(imgLarge)
      }
    </script>
  </body>
</html>
